The present invention relates to an objective optical system which is installed in a device employing multiple types of light beams having different wavelengths, such as an optical information recording/reproducing device for recording information to and/or reproducing information from multiple types of optical discs differing in recording density.
There exist various standards of optical discs (CD, DVD, etc.) differing in recording density, protective layer thickness, etc. Meanwhile, new-standard optical discs (HD DVD (High-Definition DVD), BD (Blu-ray Disc), etc.), having still higher recording density than DVD, are being brought into practical use in recent years to realize still higher information storage capacity. The protective layer thickness of such a new-standard optical disc is substantially equal to or less than that of DVD. In consideration of user convenience with such optical discs according to multiple standards, the optical information recording/reproducing devices (more specifically, objective optical systems installed in the devices) of recent years are required to have compatibility with the above three types of optical discs. Incidentally, in this specification, the “optical information recording/reproducing devices” include devices for both information reproducing and information recording, devices exclusively for information reproducing, and devices exclusively for information recording. The above “compatibility” means that the optical information recording/reproducing device ensures the information reproducing and/or information recording with no need of component replacement even when the optical disc being used is switched.
In order to provide an optical information recording/reproducing device having the compatibility with optical discs of multiple standards, the device has to be configured to be capable of forming a beam spot suitable for a particular recording density of an optical disc being used, by changing a NA (Numerical Aperture) of an objective optical system used for information reproducing/registering, while also correcting spherical aberration which varies depending on the protective layer thickness changed by switching between optical discs of different standards. Since the diameter of the beam spot can generally be made smaller as the wavelength of the beam gets shorter, multiple laser beams having different wavelengths are selectively used by the optical information recording/reproducing device depending on the recording density of the optical disc being used. For example, for DVDs, a laser beam with a wavelength of approximately 660 nm (so-called red laser light) shorter than approximately 790 nm for CDs (so-called near-infrared laser light) is used. For the aforementioned new-standard optical discs, a laser beam (e.g., so-called “blue laser” around 408 nm) with a wavelength still shorter than that for DVDs is used in order to deal with the extra-high recording density.
Examples of objective optical systems for suitably converging the multiple types of light beams onto respective ones of the multiple types of optical discs are disclosed in Japanese Patent Provisional Publication Nos. 2006-164498A (hereafter, referred to as document #1), 2006-134366A (hereafter, referred to as document #2) and 2006-12394A (hereafter, referred to as document #3).
The objective optical system disclosed in document #1 is configured such that at least one of optical surfaces of an objective lens and an optical element located on the front side of the objective lens is provided with a diffraction surface. The diffraction surface is configured such that the diffraction order at which the diffraction efficiency is maximized for the blue laser light is an even order. Each of the blue laser and the red laser is incident on the objective optical system as a collimated beam, and the near-infrared light is incident on the objective optical system as a non-collimated beam (a diverging beam). As described above, the objective optical system disclosed in document #1 achieves the compatibility with the multiple types of optical discs by appropriately selecting the diffraction effect and the degree of divergence of a light beam.
The objective optical system disclosed in document #2 has an objective optical element and a temperature compensation element. The objective optical element is provided with a plurality of types of diffraction surfaces. By this structure, the objective optical system achieves both of the compatibility with the multiple types of optical discs including the high density optical disc (e.g., HD DVD) and the temperature compensation. In contrast to the objective optical system disclosed in document #1, each of the blue laser light, the red laser light and the infra-red laser light is incident on the objective optical system as a substantially collimated beam.
Document #3 discloses an objective optical system including an objective lens or an optical element which is formed of two types of elements made of different types of materials joined together. The objective lens or the optical element is provided with a diffraction structure on a joint surface. The structure of the objective optical system of document #3 aims to achieve the high level of use efficiency of light for all of the multiple types of laser beams through utilization of the diffraction effect and the difference between refractive indexes of the materials.
However, the objective optical systems disclosed in document #1-#3 have the following drawbacks. The objective optical system disclosed in document #1 employs the diffraction structure configured such that the diffraction order at which the diffraction efficiency is maximized for the blue laser is an even order. Therefore, for at least one of the multiple types of laser beams, it is necessary to enter the laser beam to the objective optical system as a non-collimated laser beam. If a non-collimated laser beam is incident on the objective optical system, off-axis aberrations, such as a coma, may occur when an objective lens shifts in a plane perpendicular to an optical axis of the objective lens for a tracking operation.
In the objective optical system disclosed in document #2, each of the blue laser, the red laser, the near-infrared laser is incident on the objective optical system as a collimated beam. Therefore, occurrence of off-axis aberrations (e.g., a coma) can be suppressed. However, when such a configuration where each of the multiple types of laser beams is incident on the objective optical system as a collimated beam is adopted, the diffraction efficiency may decrease for at least one of the multiple types of laser beams depending on the relationship between the wavelength of each laser beam and the diffraction effect.
The protective layer of CD is thicker than those of the other types of optical discs. Therefore, when CD is used, a working distance between a record surface of the optical disc and a surface of the objective lens adjacent to the record surface becomes short relative to the working distance defined for the other types of optical discs. Therefore, when CD is used, it is required to lengthen the back focus of the objective optical system to secure an adequate working distance. However, in the objective optical system of document #2, each of the multiple types of laser beams is incident on the objective optical system as a collimated beam. In this case, the back focus of the objective optical system becomes shorter relative to a configuration where a diverging beam is incident on the objective optical system. Therefore, if the configuration of document #2 is adopted, a possibility that the objective lens contacts the optical disc (CD) arises.
The configuration of the objective optical system of document #3 requires additional manufacturing processes for joining different materials together and for appropriately forming the diffraction structure on the joint surface, and such additional manufacturing processes are required to be performed with a high degree of accuracy. As a result, manufacturing cost increases.